Trophic ecology of a large-bodied marine predator, bluntnose sixgill shark Hexanchus griseus, inferred using stable isotope analysis

Abstract

Identifying feeding patterns of large-bodied predators is necessary for predicting their potential effects on food web dynamics. However, diet information from stomach contents can be impractical to obtain because required sample sizes can be prohibitively large. In contrast, diet estimates obtained using Bayesian stable isotope mixing models require less sampling effort and can also reveal both population- and individual-level variation in diet. Here, we used an extensive stable isotope data set to evaluate the trophic role of bluntnose sixgill shark (Hexanchus griseus), a globally distributed species and among the largest sharks in the North Pacific. In total, 43 subadult sixgill sharks were sampled from Puget Sound, Washington, USA. Mixing model results indicated that the population feeds primarily on benthic fish and invertebrates (estimated median diet percentages: 33 and 35%, respectively). Further, the model indicated low individual variation in diets and that the feeding behavior of both individuals and the population as a whole tended towards generalism. Specifically, sixgill sharks appear to feed on prey groups approximately in proportion to their average biomass densities in the Puget Sound food web. As generalists, sixgill sharks are less likely to be affected by changes in the abundance of any single prey resource, and our results suggest they are unlikely to be important predators to at least some species of management concern. In addition, stable isotope data obtained opportunistically from an adult sixgill shark supports previously suggested ontogenetic movement patterns, whereby some adults make brief migrations into Puget Sound from outer coastal habitats, likely to birth, and pups feed, grow, and remain resident in Puget Sound for several years. Our findings provide insights into the trophic role of this important but understudied species and demonstrate how stable isotope analyses can further understanding of shark ecology.